Low power consumption liquid crystal display devices utilize thin film transistors (hereinafter referred to as TFTs) as their driving elements because of their high performance characteristics such as high contrast and high response speed. Low power consumption liquid crystal display devices are frequently used in, among other things, personal computers (PCs), portable televisions (TVs) and the like, and thus, the market for TFTs has expanded markedly.
Most TFTs utilize amorphous silicon (hereinafter referred to as a-Si) or poly-silicon (hereinafter referred to as p-Si) as a semiconductor for a channel layer. The a-Si is a pure silicon material constituted by a plurality of silicon crystal grains without specific crystalline direction. However, the p-Si is a pure silicon material constituted by a plurality of small single crystal silicon grains with different crystalline direction, i.e., the p-Si is a pure silicon between the single crystal silicon and the a-Si. Electron mobility is greater in p-Si than in a-Si.
The deposition of the a-Si and p-Si layers includes nucleation, growth of crystal grain, gather of crystal grain, and growth of deposition film. Specifically, the deposition process of the silicon layer includes the following steps: attracting atoms; diffusing the attracted atoms on a surface of the film; and gathering the attracted atoms at the proper step or neck location, thereby gradually forming a thin film and then growing the thin film. The driving power of the crystal grain growth, the crystal grain gather and the deposition film growth depends on the surface free energy, which gradually decreases with crystalline grain growth. The diffusion of attracted atoms on the surface of the silicon layer is related to the surface temperature. Higher surface diffusion occurs with higher the surface temperature, and consequently, it is easier to gather attracted atoms to an ideal location for growing large crystalline grains at higher surface temperatures than lower surface temperatures. Accordingly, there is a positive correlation between the size of crystal grain and the surface temperature. Furthermore, electron mobility increases with increased crystalline grain size.
U.S. Pat. No. 6,436,745, entitled “Method of Producing A Semiconductor Device”, discloses a method of producing a semiconductor device and is incorporated herein by reference. The method includes the following steps of: crystallizing an amorphous silicon film or a partially crystalline amorphous silicon film using a catalytic metal element promoting crystallization of silicon to form a crystalline first silicon film; forming a second silicon film containing a group consisting V element directly on an entire surface of the first silicon film; subjecting the first silicon film and the second silicon film to a heat treatment to thereby gettering at least some of the catalytic metal element from the first silicon film to the second silicon film; and removing the second silicon film to which the catalytic metal element has been gettered. However, the semiconductor device utilizes the a-Si as a semiconductor for a channel layer, and thus has a lower mobility of electrons.
U.S. Pat. No. 6,436,745 utilized p-Si as a semiconductor for a channel layer. Referring to FIGS. 1a and 1b, a p-Si layer 10 is generally deposited on a substrate 12 by using a plasma chemical vapor deposition process and is patterned by using photolithography and etching processes, and then is crystallized to form a conventional channel layer 14 of p-Si by using a laser anneal process. However, generally the p-Si layer is kind of rectangular in shape. The entire surface temperature is uniformly decreased. Thus, thermal energy is not easily retained after the laser anneal process. Consequently, it is not easy for the p-Si crystal grain to be grown to more than 100 micrometer.
In conventional channel layers such as the channel layer 14 of FIGS. 1a and 1b, nucleation can occur at several points in the channel layer 14. When there are many locations of nucleation, then there are many crystal grains, and consequently, the size of the crystal grains are small. When there are many crystal grains, then there are many grain boundaries, which decreases electron mobility.
Accordingly, there exists a need for a thin film transistor capable of having the big size of crystal grain and the higher mobility of electrons.